The present invention relates to plate fin heat exchangers and more particularly to fin stock materials adapted for forming fins of superhigh pressure service heat exchangers which are assembled integrally by brazing.
Heretofore, AA 3003 alloy (throughout the specification, aluminum alloy numbers all represent Aluminum Association designation unless otherwise indicated) has been extensively used as fin stock material for plate fin heat exchangers constructed from aluminum alloys by brazing, because of its good brazability. However, in the case where the heat exchanger is designed for use under superhigh pressures of at least 55 kg/cm.sup.2 G, the AA 3003 alloy can not give a satisfactory utility in the intended use because of insufficient tensile strength. Therefore, for use under such superhigh pressures, AA 3004 alloy has been used as fin stock material instead of the AA 3003 alloy. The AA 3004 alloy has a higher strength of approximately one and one-half times that of the AA 3003 alloy and exhibits a sufficient formability as fin stock material.
Fins made of the AA 3004 alloy are ordinary brazed at a temperature range of 580.degree. to 610.degree. C., using an aluminum-silicon brazing alloy with a silicon content of about 6.8 to 13 wt. %. Particularly, in cases of highly dense heat exchangers in which thicker and finely corrugated fins are incorporated with a view to enlarging their size and increasing their strength for higher pressure service, a very long time including preheating time will be required to uniformly heat all the parts to be brazed to the brazing temperature. Thus, in parts which are heated to the brazing temperatures in a relatively short time and are in contact with molten brazing alloy, the brazing alloy is placed in a liquid state for a longer time and in this time, an unfavorable excessive diffusion of silicon of the brazing material into the fins is apt to occur. As such silicon diffusion proceeds, the width of brazed joint will be progressively reduced or bonding strength will be seriously decreased. For example, in the practically used heat exchanger assemblies, the width of brazed joints is decreased to a level below 30% of the initial width of brazed joints and some assemblies can not withstand testing pressures and fracture or breakage occurs at the brazed joint portion.
Further, it has been well known that about 1% Mg contained in the AA 3004 alloy accelerates the above detrimental silicon diffusion.
Further, when brazing is carried out in a vacuum, the evaporation of Mg in fins is unavoidably caused and thereby even if Mg is present in the same level as in the AA 3004 alloy, significant strengthening effect can not be expected in fins having been subjected to vacuum brazing.